Flexographic printing element having an IR ablatable layer and process for making a flexographic printing plate

ABSTRACT

A photosensitive printing element having an IR ablatable layer and at least one barrier layer as set forth within and a process for making a flexographic printing plate from such an element are disclosed.

FIELD OF THE INVENTION

This invention relates to a photosensitive printing element and, moreparticularly, to a flexographic element having an infrared ablatablelayer capable of being selectively removed by a laser beam. Thisinvention also relates to a process for making a flexographic printingplate from such an element.

BACKGROUND OF THE INVENTION

Flexographic printing plates are well known for use in letterpressprinting, particularly on surfaces which are soft and easily deformable,such as packaging materials, e.g., cardboard, plastic films, etc.Flexographic printing plates can be prepared from photopolymerizablecompositions, such as those described in U.S. Pat. Nos. 4,323,637 and4,427,749. The photopolymerizable compositions generally comprise anelastomeric binder, at least one monomer and a photoinitiator.Photosensitive elements generally have a photopolymerizable layerinterposed between a support and a coversheet or multilayer coverelement. Upon imagewise exposure to actinic radiation, polymerization,and hence, insolubilization of the photopolymerizable layer occurs inthe exposed areas. Treatment with a suitable solvent removes theunexposed areas of the photopolymerizable layer leaving a printingrelief which can be used for flexographic printing.

Imagewise exposure of a photosensitive element requires the use of aphototool which is a mask having clear and opaque areas covering thephotopolymerizable layer. The phototool prevents exposure andpolymerization in the opaque areas. The phototool allows exposure toradiation in the clear areas so that these areas polymerize and remainon the support after the development step. The phototool is usually aphotographic negative of the desired printing image. If corrections areneeded in the final image a new negative must be made. This is atime-consuming process. In addition, the phototool may change slightlyin dimension due to changes in temperature and humidity. Thus, the samephototool, when used at different times or in different environments,may give different results and could cause registration problems.

Thus, it would be desirable to eliminate the phototool by directlyrecording information on a photosensitive element, e.g., by means of alaser beam. The image to be developed could be translated into digitalinformation and the digital information used to place the laser forimaging. The digital information could even be transmitted from adistant location. Corrections could be made easily and quickly byadjusting the digitized image. In addition, the digitized image could beeither positive or negative, eliminating the need to have bothpositive-working and negative-working photosensitive materials, orpositive and negative phototools. This saves storage space and, thus,reduces cost. Another advantage is that registration can be preciselycontrolled by machine during the imaging step. Digitized imaging withouta phototool is particularly well-suited for making seamless, continuousprinting forms.

In general, it has not been very practical to use lasers to image theelements which are used to prepare flexographic printing plates. Theelements have low photosensitivity and require long exposure times evenwith high powered lasers. In addition, most of the photopolymerizablematerials used in these elements have their greatest sensitivity in theultraviolet range. While UV lasers are known, economical and reliable UVlasers with high power are generally not available. However, non-UVlasers are available which are relatively inexpensive, and which have auseful power output and which can be utilized to form a mask image ontop of flexographic printing elements.

SUMMARY OF THE INVENTION

The present invention relates to a photosensitive printing element usedfor preparing flexographic printing plates comprising

(a) a support,

(b) a photopolymerizable layer comprising an elastomeric binder, atleast one monomer and an initiator having sensitivity to non-infraredactinic radiation, said layer being soluble, swellable or dispersible ina developer solution prior to exposure to actinic radiation,

(c) at least one barrier layer which is soluble, swellable, dispersibleor liftable in the developer solution for the photopolymerizable layerprior to exposure to actinic radiation, and

(d) at least one layer of infrared radiation sensitive material which issubstantially opaque to actinic radiation wherein the infrared-sensitivematerial is ablatable from the surface of the barrier layer uponexposure to infrared laser radiation.

The invention further relates to a process for making a flexographicprinting plate, which comprises:

(1) imagewise ablating layer (d) of the element described above withinfrared laser radiation to form a mask;

(2) overall exposing the photosensitive element to actinic radiationthrough the mask; and

(3) treating the product of step (2) with at least one developersolution to remove (i) the infrared-sensitive material which was notremoved during step (1), (ii) the areas of the barrier layer which werenot exposed to actinic radiation, and (iii) the areas of thephotopolymerizable layer (b) which were not exposed to actinicradiation.

DETAILED DESCRIPTION OF THE INVENTION

The element and process of the invention combine the convenience andsensitivity of infrared laser imaging with conventionalphotopolymerizable compositions to produce flexographic printing plateswith known good printing quality quickly, economically, and by digitalimaging means.

The photosensitive element of the invention comprises, in order, asupport, a photopolymerizable layer, at least one barrier layer, and alayer of infrared radiation sensitive material.

The support can be any flexible material which is conventionally usedwith photosensitive elements used to prepare flexographic printingplates. Examples of suitable support materials include polymeric filmssuch those formed by addition polymers and linear condensation polymers,transparent foams and fabrics. A preferred support is a polyester film;particularly preferred is polyethylene terephthalate. The supporttypically has a thickness from 2 to 10 mils (0.0051 to 0.025 cm), with apreferred thickness of 3 to 8 mils (0.0076 to 0.020 cm).

As used herein, the term "photopolymerizable" is intended to encompasssystems which are photopolymerizable, photocrosslinkable, or both. Thephotopolymerizable layer comprises an elastomeric binder, at least onemonomer and an initiator, where the initiator has a sensitivity tonon-infrared actinic radiation. In most cases, the initiator will besensitive to visible or ultraviolet radiation. Any photopolymerizablecompositions which are suitable for the formation of flexographicprinting plates can be used for the present invention. Examples ofsuitable compositions have been disclosed, for example, in Chen et al.,U.S. Pat. No. 4,323,637, Gruetzmacher et al., U.S. Pat. No. 4,427,749and Feinberg et al., U.S. Pat. No. 4,894,315.

The elastomeric binder can be a single polymer or mixture of polymerswhich can be soluble, swellable or dispersible in aqueous, semi-aqueousor organic solvent developers. Binders which are soluble or dispersiblein aqueous or semi-aqueous developers have been disclosed in Alles U.S.Pat. No. 3,458,311; Pohl U.S. Pat. No. 4,442,302; Pine U.S. Pat. No.4,361,640; Inoue et al., U.S. Pat. No. 3,794,494; Proskow U.S. Pat. No.4,177,074; Proskow U.S. Pat. No. 4,431,723; and Worns U.S. Pat. No.4,517,279. Binders which are soluble, swellable or dispersible inorganic solvent developers include natural or synthetic polymers ofconjugated diolefin hydrocarbons, including polyisoprene,1,2-polybutadiene, 1,4-polybutadiene, butadiene/acrylonitrile,butadiene/styrene thermoplastic-elastomeric block copolymers and othercopolymers. The block copolymers discussed in Chen U.S. Pat. No.4,323,636; Heinz et al., U.S. Pat. No. 4,430,417; and Toda et al., U.S.Pat. No. 4,045,231 can be used. It is preferred that the binder bepresent in at least an amount of 65% by weight of the photosensitivelayer.

The term binder, as used herein, encompasses core shell microgels andblends of microgels and preformed macromolecular polymers, such as thosedisclosed in Fryd et al., U.S. Pat. No. 4,956,252.

The photopolymerizable layer can contain a single monomer or mixture ofmonomers which must be compatible with the binder to the extent that aclear, non-cloudy photosensitive layer is produced. Monomers that can beused in the photopolymerizable layer are well known in the art. Examplesof such monomers can be found in Chen U.S. Pat. No. 4,323,636; Fryd etal., U.S. Pat. No. 4,753,865; Fryd et al., U.S. Pat. No. 4,726,877; andFeinberg et al., U.S. Pate. No. 4,894,315. It is preferred that themonomer be present in at least an amount of 5% by weight of thephotopolymerizable layer.

The photoinitiator can be any single compound or combination ofcompounds which is sensitive to non-infrared actinic radiation,generating free radicals which initiate the polymerization of themonomer or monomers without excessive termination. The photoinitiator isgenerally sensitive to visible or ultraviolet radiation, preferablyultraviolet radiation. It should be thermally inactive at and below 185°C. Examples of suitable photoinitiators include the substituted andunsubstituted polynuclear quinones. Examples of suitable systems havebeen disclosed in Gruetzmacher U.S. Pat. No. 4,460,675 and Feinberg etal., U.S Pat. No. 4,894,315. Photoinitiators are generally present inamounts from 0.001% to 10.0% based on the weight of thephotopolymerizable composition.

The photopolymerizable layer can contain other additives depending onthe final properties desired. Such additives include sensitizers,rheology modifiers, thermal polymerization inhibitors, tackifiers,plasticizers, colorants, antioxidants, antiozonants, or fillers.

The thickness of the photopolymerizable layer can vary over a wide rangedepending upon the type of printing plate desired. For so called "thinplates" the photopolymerizable layer can be from about 20 to 50 mils(0.05 to 0.13 cm) in thickness. Thicker plates will have aphotopolymerizable layer up to 100-250 mils (0.25 to 0.64 cm) inthickness or greater.

At least one barrier layer is interposed between the photopolymerizablelayer and the layer of infrared-sensitive material. The barrier layerserves two important functions. First, it minimizes migration ofmaterials between the photopolymerizable layer and theinfrared-sensitive layer because monomers and plasticizers can migrateover time if they are compatible with the materials in the other layer.If such migration occurs into the infrared-sensitive layer, then theinfrared sensitivity of that layer can be altered. In addition, this cancause smearing and tackifying of the infrared sensitive layer afterimaging. If there is no compatibility between the two layers there willbe no migration.

Second, the barrier layer shields the photopolymerizable layer fromatmospheric oxygen when the photopolymerizable layer is overall exposedto actinic radiation. The polymerization reactions require longerexposure times or higher intensity radiation sources, and the resultsare less reproducible when oxygen is present. It is possible to apply atemporary coversheet prior to exposure to actinic radiation or to carryout that exposure step in a vacuum frame. However, thephotopolymerizable layer is usually inherently tacky and steps must betaken to prevent the temporary coversheet or vacuum frame cover fromsticking to and/or damaging the surface of the photopolymerizable layer.The presence of a non-tacky barrier layer which minimizes the permeationof oxygen to the photopolymerizable layer addresses these problems.

The barrier layer must be substantially transparent to actinic radiationso that when the element is exposed to actinic radiation through theinfrared-sensitive layer, the radiation passes through the ablated areasof the barrier layer to the photopolymerizable layer without significantdiminution in intensity. The barrier layer should also initially (i.e.,prior to exposure to actinic radiation) be soluble, swellable ordispersible in the developer solvent for the photopolymerizable layer orit should be liftable in that solvent. By "liftable" it is meant thatthe solvent is able to lift off the barrier layer at least partiallyintact. This is so that the barrier layer will be removed by thedeveloper in at least those areas which are not exposed to actinicradiation, i.e., in those areas where the photopolymerizable layer isalso removed.

Two types of barrier layers can be used. The first type is one which isinsensitive to actinic radiation and is soluble, swellable, dispersibleor liftable in developer solutions for the photopolymerizable layer bothbefore and after exposure to actinic radiation. This type of barrierlayer is completely removed in both exposed and unexposed areas, alongwith the unexposed areas of the photopolymerizable layer, duringprocessing with the developer.

Examples of materials which are suitable for use as the barrier layer ofthis first type include those materials which are conventionally used asa release layer in flexographic printing elements, such as polyamides,polyvinyl alcohol, hydroxyalkyl cellulose, copolymers of ethylene andvinyl acetate, amphoteric interpolymers, and combinations thereof.

The second type of barrier layer is one which is soluble, swellable ordispersible in the developer solvent prior to exposure to actinicradiation, but is not affected by the developer solvent after exposureto actinic radiation. When this type of barrier layer is used it isremoved by the developer solvent only in those areas which are notexposed to actinic radiation. The barrier layer which has been exposedto actinic radiation remains on the surface of the polymerized areas ofthe photopolymerizable layer and becomes the actual printing surface ofthe printing plate.

This type of barrier layer can be photosensitive itself, i.e., containmonomer and initiator, or it can become photosensitive when in contactwith the photopolymerizable layer. This second type of barrier layer isusually a layer of an elastomeric composition. The composition canconsist simply of a nonphotosensitive elastomeric binder layer similarto the binder in the photopolymerizable layer or it can be the binder incombination with a monomer and initiator. A preferred barrier layer isan elastomeric composition comprising an elastomeric polymeric binder, asecond polymeric binder and optionally a nonmigratory dye or pigment.The elastomeric polymeric binder in the elastomeric composition isgenerally the same as or similar to the elastomeric binder present inthe photopolymer layer. Suitable compositions for the barrier layer arethose disclosed as elastomeric compositions in the multilayer coverelement described in Gruetzmacher et al., U.S. Pat. Nos. 4,427,759 and4,460,675.

It is also possible to use more than one barrier layer. For example, anelastomeric barrier layer may be present next to the photopolymerizablelayer and this, in turn, may be overcoated with a barrier layer which issoluble both before and after exposure to actinic radiation. The exactchoice of barrier layer(s) will depend on the nature of thephotopolymerizable layer and the infrared-sensitive layer and otherphysical requirements of the printing element.

The barrier layer, of either type, should be thick enough to act as aneffective barrier to prevent migration and air permeation and also bethin enough to minimize its effect on the exposure of thephotopolymerizable layer to actinic radiation. In general, the barrierlayer will have a thickness in the range of 0.01 to 3 mils (0.00025 to0.076 mm). A preferred thickness range is 0.015 to 2.5 mils (0.00038 to0.064 mm).

Over the barrier layer, there is at least one layer of infraredradiation sensitive material which must be ablatable, i.e., vaporized orablated, by exposure to infrared laser radiation.

The infrared-sensitive layer should be capable of absorbing infraredradiation and should be opaque to actinic radiation. This can beaccomplished using a single material or a combination of materials.Also, a binder can be present if desired. This layer may be referred toas the "infrared-sensitive layer" or the "actinic radiation opaquelayer" (radiation opaque layer). Although the infrared-sensitive layeris referred to herein as a single layer, it will be understood that twoor more infrared-sensitive layers can be used. The properties of theinfrared-sensitive layer can be modified by using other ingredients,such as, for example, plasticizers, pigment dispersants, surfactants,and coating aids, provided that they do not adversely affect the imagingproperties of the element.

The infrared-absorbing material should have a strong absorption in theregion of the imaging radiation, typically 750 to 20,000 nm. Examples ofsuitable infrared-absorbing materials include,poly(substituted)phthalocyanine compounds; cyanine dyes; squaryliumdyes; chalcogenopyryloarylidene dyes; bis(chalcogenopyrylo)polymethinedyes; oxyindolizine dyes; bis(aminoaryl)polymethine dyes; merocyaninedyes; croconium dyes; metal thiolate dyes; and quinoid dyes. Alsosuitable are dark inorganic pigments such as carbon black, graphite,copper chromite, chromium oxides and cobalt chrome aluminate; metalssuch as aluminum, copper or zinc; and alloys of bismuth, indium andcopper. The metallic materials generally function as bothinfrared-absorbing material and radiation-opaque material. They aregenerally applied without a binder.

Infrared-absorbing materials can be present in any concentration whichis effective for the intended purpose. In general, for the organiccompounds, concentrations of 0.1 to 40% by weight, based on the totalweight of the layer, have been found to be effective.

Any material which prevents the transmission of actinic light to thephotopolymerizable layer can be used as the radiation-opaque material.Examples of suitable materials include dyes and pigments. As initiatorsused in the photopolymerizable layer are often sensitive to actinicradiation in the ultraviolet and/or visible region, it is preferred touse carbon black to provide UV/visible opacity. When carbon black isused it is not necessary to use an additional infrared-sensitivematerial.

The concentration of the radiation-opaque material is chosen so as toachieve the desired optical density, i.e., so that the layer preventsthe transmission of actinic radiation to the photopolymerizable layer.In general, a transmission optical density greater than 2.0 ispreferred. The concentration of radiation-opaque material which isneeded, decreases with increasing thickness of the layer. In general aconcentration of 1-70% by weight, based on the total weight of the layercan be used. It is preferred to use 2-40% by weight, based on the totalweight of the layer.

The optional binder is a polymeric material which should satisfy severalrequirements: (1) The binder should be effectively removed by the heatgenerated by the infrared-absorbing material when the layer is exposedto infrared laser radiation. (2) The binder should be removable from thesurface of the photopolymerizable layer after the infrared imaging step.This condition is met if the binder is soluble, swellable or dispersiblein the developer solvent for the photopolymerizable layer. The bindermay also be removed in a separate step, e.g., the binder can be soluble,swellable or dispersible in a second solvent which does not affect thepolymerized areas of the photopolymerizable layer. (3) The binder shouldbe one in which the other materials in the infrared-sensitive layer canbe uniformly dispersed. (4) The binder should be capable of forming auniform coating on the barrier layer. Examples of organic binders whichcan be used include self-oxidizing polymers such as nitrocellulose;non-self-oxidizing polymers such as ethylcellulose, polyacrylic acidsand the metal alkali salts thereof; thermochemically decomposablepolymers such as homopolymers and copolymers of acrylates,methacrylates, and styrene; butadiene, isoprene, and their copolymers(i.e., polymers of two or more monomers) and block copolymers withstyrene and/or olefins, pyrolyzable films such as polyvinyl alcohol,polyvinyl chloride, and polyacrylonitrile; amphoteric interpolymers; andmixtures thereof.

A dispersant is generally added when a pigment is present in theinfrared-sensitive layer in order to disperse the fine particles andavoid flocculation and agglomeration. A wide range of dispersants iscommercially available. Suitable dispersants are the A-B dispersantsgenerally described in "Use of A-B Block polymers as Dispersants forNon-aqueous Coating Systems" by H. K. Jakubauskas, Journal of CoatingTechnology, Vol. 58; Number 736; pages 71-82. Useful A-B dispersants aredisclosed in U.S. Pat. Nos. 3,684,771; 3,788,996; 4,070,388 and4,032,698. The dispersant is generally present in an amount of about 0.1to 10% by weight, based on the total weight of the layer.

A plasticizer can be added to adjust the film forming properties of thebinder. Suitable plasticizers include, for example, triphenyl phosphite,dimethyl phthalate, diethyl phthalate, dicyclohexyl phthalate,cyclohexyl benzyl phthalate, dibutoxy ethyl adipate, ethyleneglycoldibenzoate, pentaerythritol tetrabenzoate, glycerol diacetate, glycerylcarbonate, polyethylene glycol monolaurate, methyl phthalyl ethylglycolate, o,p-toluenesulfonamide, N-ethyl-p-toluenesulfonamide, andN-cyclohexyl-o,p-toluenesulfonamide. The plasticizer should be presentin an amount effective for the intended purpose which depends on theproperties of the binder, the plasticizer, and the other components ofthe layer. In general, the amount of plasticizer, when present, is 1-30%by weight, based on the weight of the layer.

The thickness of the infrared-sensitive layer should be in a range tooptimize both sensitivity and opacity. The layer should be thin enoughto provide good sensitivity, i.e., the infrared-sensitive layer shouldbe removed rapidly upon exposure to infrared laser radiation. At thesame time, the layer should be thick enough so that the areas of thelayer which remain on the photopolymerizable layer after imagewiseexposure effectively mask the photopolymerizable layer from actinicradiation. In general, this layer will have a thickness from about 20Angstroms to about 50 micrometers. It is preferred that the thickness befrom 40 Angstroms to 40 micrometers.

The photosensitive element of the invention can also include a temporarycoversheet on top of the infrared-sensitive layer. The purpose of thecoversheet is to protect the infrared-sensitive layer during storage andhandling. It is important that the coversheet be removed prior toexposing the infrared-sensitive layer to infrared laser radiation.Examples of suitable materials for the coversheet include thin films ofpolystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymers,polyamide or polyester, which can be subbed with release layers.

The photosensitive element of the invention is generally prepared byfirst preparing the photopolymerizable layer on the support and thenapplying the barrier and infrared-sensitive layers by coating orlamination techniques.

The photopolymerizable layer itself can be prepared in many ways byadmixing the binder, monomer, initiator, and other ingredients. It ispreferred that the photopolymerizable mixture be formed into a hot meltand then calendered to the desired thickness. An extruder can be used toperform the functions of melting, mixing, deaerating and filtering thecomposition. The extruded mixture is then calendered between the supportand a temporary coversheet or a temporary coversheet which has beenpreviously coated with the barrier layer. In the latter case it isarranged so that the barrier layer is next to the photopolymerizablelayer during the calendering process. The adhesion between the barrierlayer and the temporary coversheet should be low, so that the barrierlayer will remain intact on the photopolymerizable layer when thetemporary coversheet is removed. Alternatively, the photopolymerizablematerial can be placed between the support and the temporary coversheetor the barrier layer coated temporary coversheet in a mold. The layersof material are then pressed flat by the application of heat and/orpressure.

The infrared-sensitive layer is generally prepared by coating theinfrared-sensitive material onto a second temporary coversheet. Theinfrared-sensitive layer can be applied using any known coatingtechnique including spray coating. It also can be applied by vapordeposition under vacuum or by sputtering. The last methods areparticularly useful for metal layers.

The adhesion of this second temporary coversheet should also be low sothat the sheet is easily removed. The infrared-sensitive layer can thenbe overcoated with the barrier layer or the two layers can be coatedsimultaneously.

The final element is prepared by (1) removing the temporary coversheetfrom the photopolymerizable layer and placing it together with thesecond element (second temporary coversheet/infrared-sensitivelayer/barrier layer) such that the barrier layer is adjacent to thephotopolymerizable layer; or (2) removing the temporary coversheet fromthe barrier layer on the photopolymerizable layer and placing ittogether with the second element (second temporarycoversheet/infrared-sensitive layer) such the infrared-sensitive layeris adjacent to the barrier layer. This composite element is then pressedtogether with moderate pressure. The second temporary coversheet canremain in place for storage, but must be removed prior to IR laserimaging.

Alternatively, the three layers can all be prepared on temporarycoversheets: the photopolymerizable layer by extrusion and calenderingor pressing in a mold; the barrier and infrared-sensitive layers bycoating. The final element is prepared by removing the temporarycoversheet from the photopolymerizable element, applying the barrierlayer such that the barrier layer is adjacent to the photopolymerizablelayer, removing the temporary coversheet from the barrier layer, andapplying the infrared-sensitive layer such that the infrared-sensitivelayer is adjacent to the barrier layer. The composite structure islaminated together as each new layer is added or one time for all thelayers. The temporary coversheet on the infrared-sensitive layer canremain in place for storage, but must be removed prior to imaging.

The infrared-sensitive layer can also be coated directly onto thebarrier layer which is on the photopolymerizable layer.

The process of the invention involves (1) imagewise ablating layer (d)of the element described above to form a mask; (2) overall exposing thephotosensitive element to actinic radiation through the mask; and (3)treating the product of step (2) with at least one developer solutiontreating the product of step (2) with at least one developer solution toremove (i) the infrared-sensitive material which was not removed duringstep (1), (ii) the areas of the barrier layer which were not exposed toactinic radiation, and (iii) the areas of the photopolymerizable layer(b) which were not exposed to actinic radiation.

The first step in the process of the invention is to imagewise ablatelayer (d) to form a mask. This exposure is given to the side of thephotosensitive element bearing the infrared-sensitive layer. If atemporary coversheet is present in the element, it should be removedprior to the exposure step. The exposure can be carried out usingvarious types of infrared lasers. Diode lasers emitting in the region of750 to 880 nm offer substantial advantages in terms of their small size,low cost, stability, reliability, ruggedness and ease of modulation.Diode lasers emitting in the range of 780 to 850 nm may be used toadvantage. Such lasers are commercially available from, for example,Spectra Diode Laboratories (San Jose, Calif.). YAG lasers emitting at1060 nm are also very effective.

In the infrared imagewise ablating step, material in theinfrared-sensitive layer is removed, i.e., ablated, in the areas exposedto the infrared laser radiation. The areas exposed to laser radiation inthe infrared-sensitive layer correspond to those areas in thephotopolymerizable layer which will be polymerized to form the finalprinting plate. After laser ablation, a pattern of actinicradiation-opaque material remains on the barrier layer over thephotopolymerizable layer. The areas in which the infrared-sensitivelayer remains correspond to the areas of the photopolymerizable layerwhich will be washed out in the formation of the final printing plate.

The next step in the process of the invention is to overall expose thephotosensitive element to actinic radiation through the mask. The typeof radiation used is dependent on the type of photoinitiator in thephotopolymerizable layer. The radiation-opaque material in the infraredsensitive layer which remains on top of the barrier layer on thephotopolymerizable layer prevents the material beneath from beingexposed to the radiation and hence those areas covered by theradiation-opaque material do not polymerize. The areas not covered bythe radiation-opaque material are exposed to actinic radiation andpolymerize. Any conventional sources of actinic radiation can be usedfor this exposure step. Examples of suitable visible or UV sourcesinclude carbon arcs, mercury-vapor arcs, fluorescent lamps, electronflash units, electron beam units and photographic flood lamps. The mostsuitable sources of UV radiation are the mercury-vapor lamps,particularly the sun lamps. A standard radiation source is the Sylvania350 Blacklight fluorescent lamp (FR 48T12/350 VL/VHO/180, 115 w) whichhas a central wavelength of emission around 354 nm.

It is contemplated that the imagewise exposure to infrared radiation andthe overall exposure to actinic radiation can be carried out in the sameequipment. It is preferred that this be done using a drum--i.e., thephotosensitive element is mounted on a drum which is rotated to allowfor exposure of different areas of the element.

The actinic radiation exposure time can vary from a few seconds tominutes, depending upon the intensity and spectral energy distributionof the radiation, its distance from the photosensitive element, and thenature and amount of the photopolymerizable composition. Typically amercury vapor arc or a sunlamp is used at a distance of about 1.5 toabout 60 inches (3.8 to 153 cm) from the photosensitive element.Exposure temperatures are preferably ambient or slightly higher, i.e.,about 20° to about 35° C.

The process of the invention usually includes a back exposure orbackflash step. This is a blanket exposure to actinic radiation throughthe support. It is used to create a shallow layer of polymerizedmaterial, or a floor, on the support side of the photopolymerizablelayer and to sensitize the photopolymerizable layer. The floor providesimproved adhesion between the photopolymerizable layer and the support,helps highlight dot resolution and also establishes the depth of theplate relief. The backflash exposure can take place before, after orduring the other imaging steps. It is preferred that it take place justprior to the imagewise exposure to infrared laser radiation on theinfrared-sensitive layer side of the element.

Any of the conventional radiation sources discussed above can be usedfor the backflash exposure step. Exposure time generally range from afew seconds up to about a minute.

Following overall exposure to UV radiation through the mask formed bythe actinic radiation-opaque material, the image is developed by washingwith a suitable developer. Development is usually carried out at aboutroom temperature. The developers can be organic solvents, aqueous orsemi-aqueous solutions. The choice of the developer will depend on thechemical nature of the photopolymerizable material to be removed.Suitable organic solvent developers include aromatic or aliphatichydrocarbon and aliphatic or aromatic halohydrocarbon solvents, ormixtures of such solvents with suitable alcohols. Other organic solventdevelopers have been disclosed in published German Application 38 28551. Suitable semi-aqueous developers usually contain water and a watermiscible organic solvent and an alkaline material. Suitable aqueousdevelopers usually contain water and an alkaline material. Othersuitable aqueous developer combinations are described in U.S. Pat. No.3,796,602.

Development time can vary, but it is preferably in the range of about 2to 25 minutes. Developer can be applied in any convenient manner,including immersion, spraying and brush or roller application. Brushingaids can be used to remove the unpolymerized portions of thecomposition. However, washout is frequently carried out in an automaticprocessing unit which uses developer and mechanical brushing action toremoved the unexposed portions of the plate, leaving a reliefconstituting the exposed image and the floor.

A pre-development step may be necessary if the infrared-sensitive layeris not removable by the developer solvent. An additional developer,which does not effect the polymerized photosensitive material can beapplied to remove the infrared-sensitive layer first. This isparticularly true when metallic materials are used. In such cases,etching solvents are used, such as 2% aqueous KOH solution.

Following solvent development, the relief printing plates are generallyblotted or wiped dry, and then dried in a forced air or infrared oven.Drying times and temperatures may vary, however, typically the plate isdried for 60 to 120 minutes at 60° C. High temperatures are notrecommended because the support can shrink and this can causeregistration problems.

Most flexographic printing plates are uniformly post-exposed to ensurethat the photopolymerization process is complete and that the plate willremain stable during printing and storage. This post-exposure steputilizes the same radiation source as the main exposure.

Detackification is an optional post-development treatment which can beapplied if the surface is still tacky, such tackiness not generallybeing removed in post-exposure. Tackiness can be eliminated by methodswell known in the art, such as treatment with bromine or chlorinesolutions. Such treatments have been disclosed in, for example,Gruetzmacher U.S. Pat. No. 4,400,459, Fickes et al., U.S. Pat. No.4,400,460 and German Patent 28 23 300. Detackification can also beaccomplished by exposure to radiation sources having a wavelength notlonger than 300 nm, as disclosed in European Published PatentApplication 0 017927 and Gibson U.S. Pat. No. 4,806,506.

These elements can be used to particular advantage in the formation ofseamless, continuous printing elements. The photopolymerizable flatsheet elements can be reprocessed by wrapping the element around acylindrical form, usually a printing sleeve or the printing cylinderitself, and fusing the edges together to form a seamless, continuouselement. In a preferred method, the photopolymerizable layer is wrappedaround the cylindrical form and the edges joined. One process forjoining the edges has been disclosed in German patent DE 28 44 426. Thephotopolymerizable layer can then be spray coated with, at least onebarrier layer and then with at least one infrared-sensitive layer.

Continuous printing elements have applications in the flexographicprinting of continuous designs such as in wallpaper, decoration and giftwrapping paper. Furthermore, such continuous printing elements arewell-suited for mounting on conventional laser equipment. The sleeve orcylinder on which the printing element is wrapped when the edges arefused, can be mounted directly into the laser apparatus where itfunctions as the rotating drum during the laser exposure step.

Unless otherwise indicated, the term "flexographic printing plate orelement" encompasses plates or elements in any form suitable forflexographic printing, including, but not limited to, flat sheets andseamless continuous forms. All publications/references mentioned hereinare hereby incorporated by reference unless otherwise indicated.

EXAMPLES Example 1

This examples illustrates the preparation of a photosensitive elementhaving a layer which is sensitive to infrared radiation and two types ofbarrier layers.

An infrared sensitive layer was obtained by using an infrared-sensitiveUV opaque film having a support (LaserMask™, made by James RiverGraphics, Inc., South Hadley, Mass.). A photopolymerizable layer wasobtained by using a Cyrel® 107 PLS+ printing element (E. I. du Pont deNemours and Company, Wilmington, Del.). In the printing element, thephotopolymerizable layer is overcoated with an elastomeric layer, whichfunctions as one barrier layer, and which is further overcoated with apolyamide release layer, which functions as a second barrier layer.

A sheet of the infrared-sensitive UV opaque film was sprayed with amixture of methanol and ethanol (2:1 w/w) to soften the coating. TheCyrel® 107 PLS+ coversheet was removed and the softened coating side ofthe infrared-sensitive film was placed on top of the release layer. Thiswas laminated at room temperature to squeeze out the excess solvent. TheIR support was then removed from the infrared-sensitive layer and theelement was air dried. The density of the infrared-sensitive layer onthe element was increased by laminating additional infrared-sensitivefilms, with the coating softened, onto the element four more times.

Example 2

This example illustrates the use of a Nd:YAG laser for the imaging stepin the process of the invention.

The laser used was a Quanta DCR-11 model (Spectra Physics Corp.,Mountain View, CA) at a wavelength of 1064 nm. The laser was Q-switchedwith a 20 ns pulse.

A photosensitive element prepared as described in Example 1 was given50, 100, 250 and 500 mj exposures in a shadow dot pattern with the abovelaser, using one pulse per shadow dot. Holes were ablated in theinfrared-sensitive layer resulting in a pattern of rows of shadow dotswith a distance of 250 micrometers between the dots.

After imagewise laser ablating the infrared sensitive layer, the elementwas given a backflash exposure for 50 seconds on a Cyrel® 3040 lightsource (E. I. du Pont de Nemours and Company, Wilmington, Del.), andthen given a top exposure, i.e., through the imaged infrared-sensitivecoating, for 10 minutes using the same light source without a vacuum.The exposed element was developed in a Cyrel® rotary processor for 5.5minutes using 3:1 mixture (vol/vol) of Perclene and butanol. The blackmask and the polyamide barrier layer were removed in the developer. Thephotopolymerizable layer and the elastomeric barrier layer were removedin the unexposed areas only. The plate was oven dried for two hours at60° C. and then simultaneously post exposed and light finished in aCyrel® light finishing unit for 10 minutes. Highlight dots were obtainedfor all exposures except the 50 mj exposure.

Example 3

This example illustrates the process of the invention using a diodelaser for the imagewise laser ablating step.

A photosensitive element prepared as described in Example 1 was imagedusing a Crosfield 645 scanner modified with an IR diode laser headhaving 780 to 840 nm output as described in Kellogg et al., Journal ofImaging Science and Technology, Vol. 36, No. 3, pages 220-224 (May/June1992), the disclosure of which is hereby incorporated by reference. Themounted assembly was given an imagewise exposure using signals sent fromthe Crosfield 645 reader. A halftone image was used (150 lines per inchscreen) and the exposure energy was 1200 mj/cm². The black, UV-opaquematerial was removed in the areas which had been exposed to the laser.

After imagewise laser ablating the infrared sensitive layer, the elementwas exposed and developed as described in Example 2. An image wasobtained.

Example 4

This example illustrates the preparation of a different infraredsensitive layer which is used with a single barrier layer which iscompletely removed in the developer solvent.

S-B-S, a styrene-butadiene-styrene block copolymer (Kraton® 1102, ShellChemical Co., Houston, Tex.) was precompounded with carbon black to alevel of 10 phr in a Moriyama batch mixer. An infrared sensitivecomposition was prepared by dispersing and dissolving the followingcomponents in methylene chloride as a 15% solution:

    ______________________________________                                        Component         Amount (g)                                                  ______________________________________                                        S-B-S, 10 phr carbon                                                                            33.0                                                        MABS.sup.a        16.5                                                        BHT.sup.b         0.5                                                         Final % C         6.06                                                        ______________________________________                                         .sup.a MABS = tetrapolymer of                                                 methylmethacrylate/acrylonitrile/butadiene/styrene; Blendex ® 491 fro     General Electric Co., Parkersburg, WV                                         .sup.b BHT = butyrated hydroxy toluene                                   

The coversheet was removed from a Cyrel® 112 HO printing element (E. I.du Pont de Nemours and Company, Wilmington, Del.), and the infraredsensitive composition was coated onto the release layer of the Cyrel®plate, which functioned as the barrier layer, using a 10 mil (0.025 cm)doctor knife to form a 1 mil (0.0025 cm) dry coating. The UV density ofthe resulting plate was 3.61.

The element was then laser ablated as described in Example 2 except thatexposures of 100, 200, 300, 400 and 500 μj and 1 and 2 mj were used.

After imagewise laser ablating the infrared sensitive layer, the elementwas exposed and developed as described in Example 2. In the developmentstep, the black and the barrier layer are completely removed along withthe unexposed areas of the photopolymerizable layer. An image with goodrelief highlight dots was obtained with all exposures levels except 100and 200 μj.

Example 5

This example illustrates the use of a spray-coated infrared sensitivelayer. This process is particularly useful in the formation ofcontinuous printing elements.

An infrared sensitive composition was prepared by dispersing anddissolving the components given in Example 4 in toluene to form a 15%solution. Using a Jet Pak Power Unit (Spray-on Products, Inc.,Cleveland, Ohio), the infrared sensitive composition was sprayed onto aCyrel® 112 HO printing element from which the coversheet had beenremoved leaving the release layer as the barrier layer. The coating wasaccomplished in 4 passes. The toluene solvent did not attack the barrieror photopolymerizable layers and a good element was obtained.

Example 6

An infrared-sensitive composition was prepared from the following:

    ______________________________________                                        Component         Amount (g)                                                  ______________________________________                                        Methylene chloride                                                                              283                                                         S-B-S, 50 phr carbon                                                                            33                                                          MABS              16.5                                                        BHT               0.5                                                         Surfactant.sup.a  0.2 ml                                                      ______________________________________                                         .sup.a FC-430 made by 3M Company (St. Paul, MN)                          

Using a 4 mil (0.010 cm) doctor knife, the infrared sensitive solutionwas coated onto a 1 mil (0.0025 cm) sheet of silicone-release treatedMylar® polyester. The dry coating weight was 115.2 g/dm² and theresulting optical density was 4.79. After the infrared sensitive coatingwas dry, it was overcoated with a barrier layer coating of 85% polyamide(Macromelt® 6900 from Henkel Corp., Minneapolis, Minn.) and 15%amphoteric interpolymer (40% N-t-octylacrylamide, 34% methylmethacrylate, 16% acrylic acid, 6% hydroxypropyl methacrylate, and 4%t-butyl amino ethylmethacrylate), as a 10% propanol solution, using a 4mil (0.010 cm) doctor knife.

The coversheet and release layer were removed from a Cyrel® 30 CPprinting element (E. I. du Pont de Nemours and Company, Wilmington,Del.), leaving the photopolymer layer as the top layer. The infraredsensitive composite was then laminated to the printing element such thatthe barrier layer was adjacent to the photopolymer layer.

The silicone-release Mylar® coversheet was removed and the infraredsensitive layer of the element was imagewise ablated using a commerciallaser engraving apparatus equipped with a Nd:YAG laser. The element wasmounted on the exterior of a rotating drum. The laser beam was directedparallel to the axis of the drum, and was directed toward the samplesurface with a folding mirror. The folding mirror was stationary and thedrum moved parallel to its axis. The laser beam was then focused toimpinge on the sample mounted on the drum. As the drum rotated andtranslated relative to the laser beam, the sample was exposed in aspiral fashion. The laser beam was modulated with image data, i.e.,dots, lines and text characters. The laser was operated at 2 watts andthe drum was rotated at 104 rpm with a 25 micrometer advance rate. Thisresulted in a UV-opaque patterned mask on the surface of thephotopolymer element with a tonal range of 2-98% and isolated finelines, and dots resolved using a 60 lines per inch screen.

After imagewise laser ablating the infrared sensitive layer, the elementwas given a backflash exposure for 12 seconds on a Cyrel® 3040 lightsource, and then given a top exposure, i.e., through the imagedinfrared-sensitive coating, for 7 minutes using the same light sourcewithout a vacuum. The exposed element was developed in a Cyrel® rotaryprocessor for 3 minutes using 3:1 mixture (vol/vol) of Perclene andbutanol. The black mask, the barrier layer and the unexposed areas ofthe photosensitive layer were removed. The plate was oven dried for onehour at 60° C and then simultaneously post exposed and light finished ina Cyrel® light finishing unit for 5 minutes.

Printing tests were carried out with the developed plate on a Mark Andypress System 830 (Chesterfield, MO) using Film III Dense Black EC8630ink (Environmental Inks & Coatings, Morganton, NC) diluted with EIC AquaRefresh EC1296 to a viscosity of 20 seconds as measured using a Zahn #2cup. Printing was done on Hi Gloss 40FS S246 paper (Fasson, Painesville,OH). All samples were run at optimum impression as judged by theoperator at 150 feet per minute. Good printed images were obtained.

Example 7

This example illustrates the preparation of a very thininfrared-sensitive coating which can be used with the Cyrel® printingelements in the above examples.

S-B-S was precompounded with carbon black to a level of 50 phr in aMoriyama batch mixer. An infrared sensitive composition was prepared bydispersing and dissolving the following components in methylene chlorideas a 15% solution:

    ______________________________________                                        Component         Amount (g)                                                  ______________________________________                                        S-B-S, 50 phr carbon                                                                            49.5                                                        BHT                0.5                                                        ______________________________________                                    

Using a 1 mil (0.0025 cm) doctor knife, the infrared sensitive solutionwas coated onto a 1 mil (0 0025 cm) sheet of silicone-release treatedMylar® polyester. The dry coating weight was 33.7 g/dm² and theresulting optical density was 2.45.

Example 8

The procedure of Example 6 was repeated substituting a Cyrel® 67 HOprinting element (E. I. du Pont de Nemours and Company, Wilmington, DE)for the Cyrel® 30CP printing element. The composite element was exposedand developed as described in Example except that the backflash exposurewas 15 seconds, the top exposure was 9 minutes and the development timewas 6 minutes.

Using the printing conditions of Example 6, a good print image wasobtained.

What is claimed is:
 1. A photosensitive printing element used forpreparing flexographic printing plates comprising in the orderlisted:(a) a support, (b) a photopolymerizable layer comprising anelastomeric binder, at least one monomer and an initiator havingsensitivity to non-infrared actinic radiation, said layer being soluble,swellable or dispersible in a developer solution; (c) at least onebarrier layer which is soluble, swellable or dispersible or liftable inthe developer solution for the photopolymerizable layer and which issubstantially transparent to actinic radiation; and (d) at least onelayer of infrared radiation sensitive material which is substantiallyopaque to actinic radiation, wherein the infrared-sensitive material isablatable form the surface of the barrier layer upon exposure toinfrared laser radiation.
 2. An element according to claim 1 whichfurther comprises(e) a strippable coversheet which is adjacent to layer(d).
 3. An element according to claims 1 or 2 wherein carbon black ispresent in the infrared radiation sensitive layer (d).